This invention relates to a loudspeaker system and more particularly to such a system which shall reproduce sound with a high degree of faithfulness and with a minimum of distortion and is a continuation-in-part of my co-pending application Ser. No. 857,651, filed Dec. 5, 1977, now abandoned.
Heretofore, dynamic loudspeakers of the cone type, capable of reproducing sound with ample volume and with acceptable distortion, in most cases, have the speaker cone or diaphragm facing the listeners. These cone type loudspeakers may be classified into woofers which reproduce the lowest tones, mid-range speakers which reproduce the middle tones and tweeters which reproduce the highest tones. The cone diaphragms of all three types suffer structural distortion when heavy forces are exerted on them by the voice coils. Structural distortion is greatest around the voice coil and is at an objectionable level for about 15 percent of the radial distance from the outer edge of the voice coil to the outer edge of the cone diaphragm, which terminates at the inner edge of the flexible edge or hinge, if one is used. While this distortion is slight, when measured against the entire output of the speaker system, much of it is of a shrill character and occurs within the range of frequencies which the human ear hears the loudest. This distortion, which I shall hereinafter refer to as center cone distortion, has a hashy, raspy sound which intrudes particularly on the mid-range frequencies of the human voice and of such instruments as pianos, pipe organs and violins.
Heretofore, there have been many attempts to overcome this center cone distortion. In the case of the woofer, one means has been to provide an electrical crossover network which blanks out frequencies higher than 250 to 500 cps from the woofer and then to add a larger mid-range speaker capable of reproducing sound down to 250 to 500 cps. However, even with these measures, a noticeable ghost of center cone distortion will usually be present in the woofer and the larger, more expensive mid-range speaker will have center cone distortion of its own at the upper end of the band of frequencies which the human ear hears the loudest. Depending upon the specific design of the larger mid-range speaker, it would be expected to produce center cone distortion in the range of from 4,000 to 6,000 cps or possibly as low as 2,500 cps.
Efforts have also been made in some speaker systems to eliminate center cone distortion by adding dampening masses to the center area of the woofer cones. This addition of dampening mass to the woofer cone defeats the very basic principle of a low distortion woofer, which is to keep the inertia of the cone assembly as low as possible. Accordingly, such added mass will cause the woofer to distort greatly in the very low frequencies.
Many efforts to reduce distortion in cone-type mid-range and tweeter speakers have been tried with little in the way of positive results. While metal cones have been used by some, the same center cone distortion problem exists. The result of the combined effect of center cone distortion at all three levels is a high degree of listener fatigue and an artificial sound to the loudspeaker system.
Another problem with existing loudspeaker systems is poor distribution of the sound at the upper end of the frequency range covered by the woofer since higher frequencies radiate in a more linear manner and must be reflected to make them turn corners. This effect becomes a factor of importance above about 250 cps. Heretofore, many loudspeaker systems have employed woofers designed to cut off at from 1,000 to 1,500 cps. The linear nature of the frequencies which such woofers emit between 250 and 1,500 cps, causes such frequencies to form a narrow band along the axis of the speaker. These frequencies are thus poorly distributed while the frequencies below 250 cps are well distributed because of their non-linear nature. As the frequencies increase, the problem becomes progressively more severe in the mid-range and tweeter speakers.
A still further problem encountered with conventional loudspeaker systems relates to the shape of the response curve of the woofers. Ideally, the highest volume of reproduction should occur at the lowest audible frequencies. But in practice most good woofers, because of the strain in the center of the cones, have a peak or "knee" on the response curve with the highest volume of sound occurring at about 1,000 cps. This knee or peak, which is usually near the cross-over point between the woofer and the mid-range speaker, causes a shrillness which is almost impossible to defeat with the aforementioned electrical cross-over network. Since the loudest sound which the human ear hears ranges between about 2,000 and 5,000 cps, it is preferable to have the woofer cut off at no more than about 1,000 cps so that the mid-range speaker can take over at above 1,000 cps and with its very small, low inertia cone, reproduce the loudest frequencies the ear hears with the lowest possible distortion.
Another attempt to solve the problem of center cone distortion has been to strengthen the cone sufficiently to eliminate such distortion, such as by providing a sandwich-like cone having inner and outer metal members with a non-metallic layer therebetween. The Hitachi metal cone loudspeaker shown in the October 1977 issue of Stereo Review Magazine, page 12, is such a speaker. The Tannoy speaker shown in the June 1977 issue of Stereo Review, page 107, shows a speaker cone in which ribbing is employed to prevent cone break-up or distortion under stress. Another attempt to prevent center cone distortion has been to use high strength carbon filaments in the cone paper.
There are at least three factors tending to cause the radial stresses in a loudspeaker cone to increase as we progress from the outer edge of the cone toward the voice coil at the center of the cone. These factors are:
(1) The area of a cone increases as the second power of the slant height of the cone. Accordingly, the resistance of the air to the movement of the cone would increase as the second power of the slant height of the cone.
(2) Since the outer portions of the cone cause compression or tension forces along the slant height, depending on which half of the sound wave is being induced at the time, such that the inner portions of the cone must bear not only the force caused by its pushing of the air but must also bear the forces from these outer portions as well, another first power must be added.
( b 3) Since the cross section of the paper cone through which this force must be transmitted to the voice coil diminishes to the first power of the reduction in slant height, the unit stress will increase as the first power as we progress along the slant height of the cone toward the voice coil.
From the above it can be seen that the unit stress along the slant height of the woofer cone increases at least as the fourth power as we progress from the outside of the cone toward the voice coil. The combined total of the tensile or compressive forces in a paper cone about 0.010 inch thickness may exceed one pound near the center of the cone. This can induce a radial unit stress of as much as 20 pounds per square inch near the voice coil. While this may seem like a very low unit stress, it is enough to compress the inner one inch of slope distance by about 0.00025 inch. But because of the fourth power function causing the stresses to lessen drastically with each inch we move away from the voice coil, we would compress the portion of the cone one inch farther out from the voice coil by only 0.000,015,6 inch under the same conditions. From this it can be seen that the inner 3.5 inches of a typical woofer cone would distort radially by about 16 times as much as the portion of paper cone between diameters of 3.5 inches and 5.5 inches.
It will be understood that the distortion with which we are concerned can be produced in sufficient quantity to offend the human ear by compression or tension forces so minute as to be difficult to measure.
The Wolff U.S. Pat. No. 1,786,279 and the Leon U.S. Pat. No. 2,643,727 both show reflectors for speaker systems. However, both patents show the reflectors as extending all the way across the center portion of the speaker whereby the reflectors reflect substantially all of the sound output of the speaker. Accordingly, with such reflectors, there could be no discrimination of sounds emitted from the speaker. While the Thuras U.S. Pat. No. 2,037,185 and the Buchmann U.S. Pat. No. 2,714,047 both disclose a loudspeaker system having a damper-like member mounted adjacent the center of the speaker cone, these patents do not teach the use of such a damper member in combination with a reflector member mounted in position to reflect only sound from the outer portion of a speaker cone aimed at least 90.degree. away from the listeners with the innermost edge of the reflector being located at least 15 percent of the radial distance measured outwardly from the outer edge of the loudspeaker voice coil to the outer edge of the cone diaphragm.